Infants with chronic lung disease have acute episodes of hypoxemia that are often accompanied by wheezing. To test whether a sudden reduction in FIO2 might increase airway obstruction in such infants, we measured the flow-volume relationship, O2 saturation, and skin-surface CO2 tension in 19 sedated infants, 11 with chronic lung disease, and 8 control infants, before and during 10 min of continuous hypoxemia. In the infants with chronic lung disease, a 20 to 25% reduction in FIO2 caused acute hypoxemia (O2 saturation, 77 +/- 8%) and an associated decrease in mid-expiratory flow from 103 +/- 55 to 69 +/- 37 ml/s (mean +/- SD; p less than 0.05) in the absence of a significant change in tidal volume or skin-surface CO2 tension. In the infants without lung disease, breathing 17% O2 led to a significant increase in minute ventilation (26 +/- 25%; p = 0.05), but there was no consistent change in mid-expiratory flow. To further study the effects of an acute reduction in FIO2 on pulmonary function in infants with chronic lung disease, we measured lung mechanics in 6 infants and end-expiratory lung volume in 5. Baseline lung resistance was high (49 +/- 35 cm/l/s) and increased by 55 +/- 30% (p less than 0.05) in response to hypoxemia. Baseline dynamic lung compliance was low (2.5 +/- 1.5 ml/cm) and decreased by 24 +/- 10% (p less than 0.05). Functional residual capacity increased from 26 +/- 13 to 33 +/- 14 ml/kg (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)